Transparent article having protective silicon nitride film

ABSTRACT

Transparent articles comprising transparent, nonmetallic substrate and a transparent film stack is sputter deposited on the substrate. The film stack is characterized by including at least one infrared reflective metal film, a dielectric film over the metal film, and a protective silicon nitride film of 10 Å to 150 Å in thickness over the said dielectric film. The dielectric film desirably has substantially the same index of refraction as does silicon nitride and is contiguous with the silicon nitride film.

RELATED APPLICATIONS

This application is a divisional application of prior application Ser.No. 08/547,690 now U.S. Pat. No. 5,834,103, which is a continuation ofprior application Ser. No. 08/237,931 filed May 31, 1994 abandoned, bothof which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention is directed to transparent coatings for substrates andparticularly to transparent coatings that are physically and chemicallyresistant.

BACKGROUND OF THE INVENTION

Glass and similar transparent substrates can be coated with transparentfilms to alter the optical properties of the glass. High visibletransmittance, low emissivity coatings are characterized by theirability to transmit visible light while minimizing the transmittance ofother wavelengths of light, particularly light in the infrared spectrum.This characteristic is useful for minimizing radiative heat transferwithout impairing visibility, and coatings of this type find utility inarchitectural glass or automobile windows. It is often desired tomaintain reflectance relatively consistent throughout the visiblespectrum so that the coating has a “neutral” color; that is, colorless.

Generally speaking, coatings on glass that are provided for hightransmittance and low emissivity comprise a stack of films having one ormore thin metallic films with high infrared reflectance and lowtransmissivity that are disposed between antireflective dielectriclayers that commonly are metal oxide films. The metal oxide layers serveto reduce visible reflection of the film stack to enhance transmittance,and are characterized by relatively high indices of refraction, commonlyon the order of 1.9 or more.

Thin, transparent metal films of silver, copper and the like aresusceptible to corrosion (e.g., staining) when they are brought intocontact, under moist or wet conditions, with various staining agentssuch as atmosphere-carried chlorides, sulfides, sulfur dioxide and thelike. Films of this type commonly are employed on inner surfaces ofmulti-pane glass units so that the films are maintained in a drycondition by desiccants or the like that remove moisture from theinterpane spaces. Staining can occur when coated panes of glass arestored for later fabrication into insulating glass units.

Film stacks frequently are isolated from contact with the environment,and a film stack of the type described may be positioned on one of theinner surfaces of a multipane insulating glass unit. However, when glasspanes bearing coating stacks are transported or assembled into multipaneunits, they often are subjected to relatively harsh conditions which maycause physical marring of the film stacks.

Film stacks commonly are provided on glass sheets on a commercialproduction basis through the use of magnetron sputtering techniques suchas those described in Chapin, U.S. Pat. No. 4,166,018.

Gillery, et al. U.S. Pat. No. 4,786,563 suggests the use of a thinovercoat of titanium dioxide as a protective layer. Titanium oxideovercoats may be particularly prone to scratching or abrasion duringshipping and washing operations, however, rendering the glass panescommercially unsuitable for use. O'Shaughnessy, et al. U.S. Pat. No.5,296,302 corrects the problem by providing a protective overcoat of anoxide such as zinc oxide, the latter being relatively very thin incomparison to other films in the stack, and protective overcoats of thistype having thicknesses in the range of 10-40 Å are disclosed.

High transmittance, low emissivity film stacks of the type described inU.S. Pat. No. 5,296,302, despite their excellent resistance toscratching, nonetheless have experienced problems in connection with thetarnishing or other discoloration of the reflective metal layers, whichcommonly are silver. Moreover, since the sputter deposition of certainfilms such as titanium oxide proceeds more slowly than zinc oxide, forexample, it would be desirable to avoid the presence of titanium oxidefilms of thicknesses greater than, for example, 30 Å.

It would be desirable to provide such film stacks with protection notonly against physical damage (e.g., scratching) but also againsttarnishing or discoloration of the metal reflective layers employed insuch film stacks.

SUMMARY OF THE INVENTION

We have found that application of a thin sputtered-on film of a compoundof silicon and nitrogen such as silicon nitride as a protective film ina film stack provides the underlying metal (e.g., silver) metal film(s)with excellent resistance to corrosion while at the same time providingthe underlying stack with resistance to physical marring, all withoutsubstantial effect upon optical properties of the stack. Preferably, thefilm directly beneath the silicon nitride protective film is adielectric film having an index of refraction substantially the same asthat of silicon nitride, that is, about 2.0±0.1. The film directlybeneath the silicon nitride film may be of a metal oxide such as anoxide of zinc or tin or alloys thereof, these oxides havingsubstantially the same index of refraction. In this manner, the opticalproperties of a given film stack may be adjusted by adjusting thethickness of the contiguous, combined zinc oxide/silicon nitride films.Desirably, the silicon nitride film is at least 10 Å thick, and ispresent at a thickness less than 150 Å, preferably not greater than 100Å and most preferably not greater than about 50 Å. Although the filmstack may have thin (usually not greater than about 20 Å) sacrificial orshielding films of titanium compounds such as titanium oxide, the stackdesirably is free of titanium-containing films having thicknessesgreater than about 30 Å.

In one embodiment, the invention comprises a transparent support such asglass having a transparent film stack sputter deposited upon it, thefilm stack comprising a reflective metal film, a dielectric film overthe metal film, and a thin (10 Å-150 Å) protective silicon nitride filmover the dielectric film. In another embodiment, the invention relatesto a transparent article having a pair of reflective metal filmsseparated by a dielectric film, the film stack including an outerdielectric film and a thin protective film of silicon nitride over theouter dielectric film. Desirably, the dielectric film immediatelybeneath the protective silicon nitride film is a metal oxide film suchas zinc oxide or tin oxide that has an index of refraction substantiallythe same as that of the silicon nitride so that the outer dielectricfilm and the silicon nitride film can be treated as a single film foradjusting optical properties of the stack.

DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a substrate coated with a film stack ofthe invention according to one embodiment of the invention; and

FIG. 2 is a schematic view of a substrate coated with a film stackaccording to another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, a transparent article 10 is shown ascomprising a glass sheet 12 bearing a film stack 14. The film stack 14may be manufactured by any convenient method, but magnetron sputteringtechniques of the type described in U.S. Pat. No. 4,166,018, theteachings of which are incorporated herein by reference, are preferred.In this method, the glass sheet 12 is transported through a series oflow pressure zones in which the various films which make up the filmstack 14 are sequentially applied. Metallic films are sputtered frommetallic sources or “targets”. Metal oxide or metal nitride films may beformed by sputtering the metal in a reactive oxygen or nitrogenatmosphere, or by first sputtering the metal on a substrate to form afilm and then subjecting the metal film to a reactive atmosphere ofoxygen or nitrogen. If desired, two or more contiguous films ofdifferent metal oxides may be used instead of a single metal oxide film.Magnetron sputtering techniques are well known in the art and need notbe described here in detail.

With reference to FIG. 1, upon the surface 13 of the glass pane 12 issputter deposited a first antireflective film 16 of sufficient thicknessto reduce or eliminate any reflectance which may arise from theinterface between the following metal film 18 and the glass substrate12. Appropriate metal oxides for use in film 16 include zinc oxide, tinoxide, indium oxide, bismuth oxide, titanium oxide, hafnium oxide, andzirconium oxide, and combinations thereof, but in a particularlypreferred embodiment, the inner metal oxide film 16 is formed of zincoxide at a thickness in the range of 120-700 Å, and preferably in therange of 300-700 Å.

Following sputter deposition of the metal oxide film 16, a reflectivemetal film 18 such as silver, gold, copper or aluminum is sputterdeposited, the metal preferably being silver deposited at a thickness inthe range of about 70 Å to about 150 Å with a thickness of about 110 Åbeing most preferred. The resulting metallic silver film, if notprotected, will react with such reactive gasses as oxygen if notprotected. Hence, directly upon the metal film 18 is sputter deposited athin sacrificial metal film (referred to herein as a “shielding” film)such as titanium, the metal film 20 serving to shield and thus protectthe underlying silver film 18 during the sputter deposition, in thisexample, of a following metal oxide film 22 and thus preventing thesilver film from becoming oxidized. The resulting film 20 thus is formeddirectly by the application of a thin film of titanium metal directly tothe reflective silver film 18, followed by oxidation of the titaniumfilm. When the overlying metal oxide film 22 is applied by sputtering ametal in a reactive oxygen atmosphere, the titanium metal film oxidizesto form a titanium oxide and thus serves to shield the underlying silverfilm. Applying a shielding titanium film at about 20 Å has been found towork well and, as mentioned, leads to the formation of a titanium oxidefilm 20.

In the embodiment depicted in FIG. 1, the next film to be applied is adielectric antireflective film that is desirably a metal oxide and mostdesirably has an index of refraction substantially identical to that ofsilicon nitride. The indices of refraction of silicon nitride and zincoxide or tin oxide, for example, are substantially identical. Thedielectric antireflective film 22 may have a thickness in the range of150-350 Å.

Finally, over the antireflective dielectric film 22 is sputter depositeda thin film 24 of silicon nitride, the latter desirably being sputterdeposited from a silicon target (suitably doped to render it conductive)in a nitrogen atmosphere. The thin silicon nitride film can be presentin any thickness which does not unduly interfere with the opticalproperties of the sheet, and silicon nitride thicknesses in the range ofabout 10 Å to about 150 Å are appropriate. It is generally desired thatthe silicon nitride film not be thicker than about 50 Å.

If the silicon nitride film 24 and the underlying dielectric layer 22have substantially the same index of refraction and are contiguous, thenthese two films can be treated as a single film 26 for the purpose ofdetermining and adjusting the optical properties of the film stack 14.That is, the relative thicknesses of the films 22 and 24 in thisinstance will have little, if any, effect upon the optical properties ofthe film stack; rather, it is the thickness of these films combined thatcan be adjusted to provide the desired optical properties. The combinedthickness of the films 22 and 24 in this instance desirably will be inthe range of about 250 to about 400 Å. For film stacks having only asingle reflective metal film such as the stack depicted in FIG. 1, acombined thickness of films 22 and 24 of 300-350 Å is preferred, whereasfor film stacks using two or more metal reflective layers such as thestack depicted in FIG. 2, a combined thickness of films 22′ and 24′ of275-325 Å is preferred.

Referring now to FIG. 2, an embodiment similar to that of FIG. 1 isshown except that the series of films 18, 20 and 22 are repeated,although not necessarily at the same thickness. Elements similar tothose shown in FIG. 1 are given the same numbers, primed.

In FIG. 2, a transparent support 12′ such as glass is provided with afirst dielectric antireflective film 16 followed by an infraredreflective metal film 18′ such as silver. Atop the silver film issputter deposited a thin titanium metal shielding film which issubsequently oxidized to titanium oxide, then the next antireflectivedielectric film 28, in the form of, for example, zinc oxide is applied.Thereafter, a metal film 30 is applied that is similar to film 18′,followed by a titanium shielding film which is oxidized subsequently totitanium oxide and designated 32, followed by the dielectricantireflective film such as a zinc oxide film 22′. Over the dielectricfilm 22′ is placed a thin silicon nitride protective film designated24′.

Although it is desired that the silicon nitride film 24, 24′ be theoutermost or overcoat film of the film stack, it may on occasion bedesirable to add a subsequent thin, highly scratch-resistant film 34such as a film of zinc oxide as reported in U.S. Pat. No. 5,296,302, theteachings of which are incorporated herein by reference. The latter film34 desirably is applied at a thickness in the range of about 10 to about40 Å and does not significantly affect the optical properties of theremainder of the stack. The overcoat 34, although desirably of zincoxide, may be formed of an oxide of a metal selected from the groupconsisting of tin, indium, bismuth and alloys thereof.

Film stacks of the type depicted in FIGS. 1 and 2 may be sputterdeposited to form the sequential films of the type and thicknesses setout in the following Tables 1 and 2. Zinc oxide was sputter depositedfrom a zinc target in an atmosphere containing oxygen, and siliconnitride was sputter deposited from a doped silicon target in anatmosphere containing nitrogen. Of course, additional films such astitanium nitride, titanium, stainless steel and other metals, and oxidesof metals can be employed as desired, but it is desired that the filmslisted in Tables 1 and 2 be contiguous to each other.

TABLE 1 Material Thickness, Å Glass Zinc Oxide 370 Silver 110 Titanium*20 Zinc Oxide 225 Silicon Nitride 100

TABLE 2 Material Thickness, A Glass Zinc Oxide 120 Silver 110 Titanium*20 Zinc Oxide 600 Silver 110 Titanium* 20 Zinc Oxide 230 Silicon Nitride100 *Shielding film that is subsequently converted to titanium oxide

Film stacks of the type described, utilizing a silicon nitrideprotective overcoat having a thickness in the range of about 10 Å toabout 150 Å, have been demonstrated to show superior results incomparison to film stacks of the type depicted in U.S. Pat. No.5,296,302 with respect to resistance to discoloring of the metal films.In one series of tests, glass sheets with the film stacks ofapproximately the same constitution as shown in Tables 1 and 2 weresubjected to severe weatherability tests: Racks of the coated glasssheets separated by small polystyrene beads were placed in an enclosedspace and subjected to cycles of high humidity and temperature (eachperiod being 16 hours at 90° F. and 60% relative humidity) andrelatively low temperature and humidity (8 hours at 65-70° F., 30%relative humidity). After 21 days (21 cycles), the film stacks havingthe silicon nitride overcoat were visibly unchanged, whereas the otherfilm stacks exhibited commercially unacceptable spotting associated withdamage to the silver layers.

While a preferred embodiment of the present invention has beendescribed, it should be understood that various changes, adaptations andmodifications may be made therein without departing from the spirit ofthe invention and the scope of the appended claims.

What is claimed is:
 1. A transparent article comprising a transparent,non-metallic substrate and a transparent film stack deposited upon thesubstrate, said film stack comprising, in sequence, a first dielectricfilm, a metal film, a second dielectric film of a metal oxide, and aprotective film of silicon nitride having a thickness in the range of 10to 100 Å.
 2. The transparent article of claim 1 wherein the seconddielectric film has an index of refraction essentially the same as thatof silicon nitride.
 3. The transparent article of claim 1 wherein thecombined thickness of said second dielectric film and the siliconnitride protective film ranges from about 250 to 400 Å.
 4. Thetransparent article of claim 2 wherein said second dielectric film andthe silicon nitride film are contiguous.
 5. The transparent article ofclaim 4 wherein the combined thickness of said second diclectrie filmand said silicon nitride film is 300-350 Å.
 6. The transparent articleof claim 3 wherein the combined thickness of said second dielectric filmand said silicon nitride film is 275-325 Å.
 7. The transparent articleof claim 1 wherein the metal film is silver.
 8. The transparent articleof claim 7 wherein the metal film of silver is 70-100 Angstroms.
 9. Thetransparent article of claim 1 wherein said metal oxide is zinc oxide ortitanium dioxide.
 10. The transparent article of claim 1, wherein themetal film is 70-100 Angstroms.
 11. A transparent article comprising atransparent, non-metallic substrate and a transparent film stackdeposited upon the substrate, said film stack comprising, in sequence, adielectric film contiguous to the transparent substrate, a metal film, ashielding film contiguous to the metal film, a metal oxide film, and aprotective film of from 10 Å to 100 Å of silicon nitride contiguous tosaid metal oxide film.
 12. The transparent article of claim 7 whereinthe metal film is silver.
 13. The transparent article of claim 12wherein the metal film of silver is 70-100 Angstroms.
 14. Thetransparent article of claim 11 wherein the index of refraction of themetal oxide film is essentially the same as silicon nitride.
 15. Thetransparent article of claim 11 wherein the shielding film and the metaloxide film are contiguous.
 16. The transparent article of claim 11wherein the metal oxide film is zinc oxide or titanium dioxide.
 17. Thetransparent article of claim 11 wherein the metal film is 70-100Angstroms.
 18. The transparent article of claim 11 wherein the combinedthickness of said second dielectric film and the silicon nitrideprotective film ranges from about 250 to 400 Å.